This invention relates to a method and apparatus for ensuring that small screws used to hold together dental implant components are tightened to the correct initial stress level, or “preload.” According to the National Institute of Health, among the factors involved in the design of a dental implant are the forces produced during implant loading, the dynamic nature of loading, and the mechanical and structure properties of the prosthesis in stress transfer to tissues. Unfortunately, accurate data on such parameters are incomplete. National Institutes of Health Consensus Development Conference Statement on Dental Implants. June 13-IS, 1988.
During the early 1970's the dental profession was very hesitant to use dental implants or fixtures surgically implanted into a patient's jawbone as a treatment option to replace missing teeth. However, success with implants in the past 30 years has replaced this skepticism. This is due to the efforts of P-I Brånemark and co-workers in Sweden who introduced the concept of osseointegration in humans. When the principles of osseointegration are followed, the anchorage of a non-biological titanium implant unit to living bone will occur, with approximately 95% and 85% implant survival rates for the lower and upper jaws, respectively. See, for example in U.S. Pat. Nos. 4,824,372, 4,872,839 and 4,934,935 to Jorneus et al., Brajnovic and Edwards, respectively.
One of most critical aspects in the replacement of missing teeth using dental implants is the ability of small screws positioned within the implant complex to hold the various implant parts together during loading and stress transfer. As any screw in the implant system is tightened, the initial stress level developed within the screw becomes critical to the maintenance of the joint stability between the parts the screw is clamping together. Owing to the high strain level that the assembled joint experiences in everyday life, this initial stress level called the preload is of paramount importance. Insufficient tightening of a screw in the implant system can result in the screw becoming loose rather quickly, and over time this looseness can lead to fracture of the screw and potentially failure of the implant reconstruction. This is particularly critical for screws that secure spacers or abutments to the implant or fixture.
The stability of the screw joint is considered a function of the preload stress achieved in the screw when applying the preload tightening torque to clamp the implant components together. The optimum preload torque is influenced by the geometry of the screw, the contact relationships between the screw and its bore, between the screw and its threads, and between the bearing surfaces of the components clamped together by the screw, friction, and the properties of the materials used. One example is the joint formed between the bearing surface of the implant and the bearing surface of the spacer or abutment. Another example is the joint formed between a prosthesis and an abutment, also held together by a small screw in the implant system.
When the screw joint experiences instability, the screw will either loosen or fracture. Screw joint failure occurs in two stages. The first stage consists of external functional loading applied to the screw joint that gradually leads to the effective erosion of the preload in the screw joint. Any transverse or axial external force that causes a small amount of slippage between the threads releases some of the stress, and therefore, some of the preload is lost. The greater the preload applied to a screw joint (up to a maximum equal to the proportional limit), the greater the resistance to loosening and the more stable the joint. As long as the frictional forces between the threads remain large, a greater external force will be required to cause loosening.
Once the critical load exceeds the screw joint preload, it becomes unstable. The external load rapidly erodes the remaining preload and results in vibration and micromovement that leads to the screw backing out. Once this second stage has been reached, the screw joint ceases to perform the function for which it was intended and has failed.
Optimizing the preload of a screw used in a dental implant system is critical for implant screw joint stability. As was stated earlier, implant screw loosening and fractures are quite common. The fact that on average complications with implant screw will occur in one out of every four implants surgically placed is significant. The need for optimum preload in screw tightening at the initial stages of implant component assembly and completion of the final implant restoration cannot be left to chance. An instrument that scientifically records the preload established in these implant screws following tightening and prior to any external load applications is essential to implant performance and the quality of life of the patient who receives implants as part of their dental rehabilitations.
It has been reported by Patterson and Johns that to achieve the maximum preload possible in component screws for dental implants, it is necessary to apply the appropriate tightening torque to each screw. Torque tightening devices for implant screws are discussed, for example, in U.S. Pat. Nos. 6,109,150 and 5,626,474. However, most screw torque-tightening devices lack accuracy because of a number of variables beyond the control of these conventional instruments. This means that the maximum stress developed in an implant screw tightened by conventional torque-tightening devices may be less than 70% of the yield strength of the screw itself and therefore well below the maximum possible preload for a stable joint. If the screw is loaded to the appropriate preload level one can be confident that the screw will not fail during the life of a patient when “normal” external loads are applied.
Ultrasound instrumentation has been used to measure the preload established in large bolts and screws in industrial applications. Thus far, however, it has not been applied to small screws the size of those used in implant systems. In industrial applications for large bolts and screws, the most common ultrasonic instruments for control of screw tension are called “pulse-echo” or “transit time” instruments. Bickford has described the use of this method with large bolts. A drop of fluid is placed on the head of the bolt to reduce the acoustic impedance between the transducer and bolt head. An acoustic transducer of some sort is placed against the bolt head. The instrument is then zeroed for this particular bolt because each bolt will have a slightly different acoustic length even if their physical lengths are the same. The zero load is recorded before tightening. Next, the bolt is tightened. If the transducer can remain in place during tightening, it will show the buildup of stretch or tension in the bolt during tightening. If it must be removed, it is repositioned on the bolt again after tightening to show the stress level achieved. If at some future time one wishes to measure the tension present within the bolt, the original data can be input to the instrument computer unit and after placing the transducer on the top of the bolt, the instrument will record the existing tension and the zero stress conditions.
In principle, the electronic instrument delivers a voltage pulse to the transducer, which emits a brief burst of ultrasound (typically five to seven or more cycles). This burst passes down through the bolt, echoes off the far end, and returns to the transducer. The electronic instrument measures very precisely the time delay required for the burst of sound to make its round trip in the bolt. As the bolt is tightened, the amount of time required for the ultrasound to make its round trip increased for two reasons: 1) the bolt stretches as it is tightened, so the path length increases, and 2) the average velocity of sound within the bolt decreases because the average stress level has increased. At low strain those functions can be approximated by linear ones of the preload in the bolt, so the total change in transit time is also a linear function of preload.
In dental implant technology, it is important to know what preload exists in implant screw joints at any time during implant therapy and throughout the life of the implant.
All of the currently used implant screws are fabricated from materials that are nontransparent and nonmagnetic. No other efficient technique for stress measurements of nonmagnetic and nontransparent materials is available. In contrast, a magnetic hysteresis curve can be used to infer the stress in magnetic materials, and also optical coherent methods can be used to infer the stress in transparent materials. However, the accuracy of this latter method is significantly lower than that of the ultrasonic TOF measurements, and as stated the implant screws are made of nonmagnetic materials. The use of mechanical methods for stress measurements requires exact measurements of the length of the implant parts, and with the 30 plus implant manufacturers throughout the world and their reluctance to provide this data, this method has definite limitations.
Ultrasonic measurement of the stress in a screw or bolt with a relatively big cross-section and length has been known for some time. Since the early fifties the technique has been theoretically and experimentally proven for a range of materials. Experimental and theoretical results obtained by Huges and Kelly on samples of rail steel with various load conditions have shown the proportionality between the uniaxial stress and velocity of acoustic waves. However, since then the method has been used for only relatively long and large cross-section components, partially due to an insufficient accuracy of TOF measuring devices. At present a digital oscilloscope's sampling rate ranging to several gigahertz makes possible a real time measurement of time intervals with the 10-100 picosecond accuracy. As to the dental implant screw in question, the ultrasonic evaluation of the stress via the time of flight measurement in principle is feasible. In practice the method is not straightforward and several factors have the potential to influence the accuracy, however, the whole performance is predictable. Difficulties reside in the small size transducer required (around 0.5 mm. active element diameter), and the small length inducing a low variation of the time of flight of the ultrasonic pulse. The smaller the transducer, the greater the exposure to a stronger mechanical stress. The smaller the length of the screw, the less variation in the time of flight and consequently the lower precision of the stress measurement obtained. Ambient temperature influencing elastic properties of materials, could also be a concern, which can be controlled.
The optimum preloads suggested for implant screw joints are a percentage of the yield strength of the screw. For example, 50-60% of yield has been suggested for average nongasketed joints, with “normal” safety or performance concerns. A 70-75% of yield has been suggested as the upper limit for nongasketed joints where “low preload” problems have been experienced in the past such as leaks, self-loosening, fatigue, etc. Joints which have had consistent “low preload” problems in the past, and where the need to avoid failure is significant and where service loads (or ignorance of service loads) make it unwise to take the screws any closer to the yield point, a 85-95% of yield has been suggested. Obviously, the preloads suggested for various screw joints demonstrate considerable variation, and depending on the joint requirements, the amount of preload achieved (% of the yield) would be significant in the performance of the joint. Furthermore, the amount of preload suggested depends on the accuracy of knowing the yield point of the screw. McGlumphy has reported significant differences between screws from several implant manufacturers even though the suggested tightening forces, and thus the preload achieved for these screws were the same. The force needed to cause failure in abutment screws for the systems as tested by McGlumphy ranged from 1.22 to 17.23 kg. However, even if the ultimate tensile strength of the screw, the proportional limit and the elastic range were known, neither the preload created by tightening using a torquing device suggested by the manufacturer for the particular screw nor the variability in the preload as a result of the tightening instrument used by the operator is known.
In summary, it would appear that a great deal of subjectivity exists in the tightening of implant screws. It isn't any wonder that screws loosen or fracture. The tightening instruments are a major variable. The quality and quantity of the tightening torque is in question. The “target” preload is uncertain. Finally, the achieved preload is unknown. In implant joints, which are very critical joint assemblies, the stability of the joint begins with knowing the exact preload achieved following the clamping together of the components. The Preload Measurement Gage will provide clinicians with that information.